docs: document the new -f/--rcfile option

doc/nano.1

@@ -226,6 +226,10 @@ system either Backspace acts like Delete or Delete acts like Backspace.
 .BR \-e ", " \-\-emptyline
 Do not use the line below the title bar, leaving it entirely blank.
 .TP
+.BR \-f\ \fIfile ", " \-\-rcfile= \fIfile
+Read only this \fIfile\fR for setting nano's options, instead of reading
+both the system-wide and the user's nanorc files.
+.TP
 .BR \-g ", " \-\-showcursor
 Make the cursor visible in the file browser (putting it on the
 highlighted item) and in the help viewer.  Useful for braille users

doc/nano.texi

@@ -334,6 +334,11 @@ system either Backspace acts like Delete or Delete acts like Backspace.
 @itemx --emptyline
 Do not use the line below the title bar, leaving it entirely blank.
 
+@item -f @var{file}
+@itemx --rcfile=@var{file}
+Read only this @var{file} for setting nano's options, instead of reading
+both the system-wide and the user's nanorc files.
+
 @item -g
 @itemx --showcursor
 Make the cursor visible in the file browser (putting it on the